1. Field of the Invention
The present invention relates to a vacuum coating apparatus for forming a thin layer coating film of treatment fluids such as paints over the surface of a material with a long body, i.e., wood, to be treated.
2. Description of the Prior Art
As a coating apparatus for forming the thin layer coating film over the material to be coated, a brush, spray, roll coater, curtain coater, dip coater, blade coater, and the like are well known. When applying treatment fluids over the material surface, however, such known apparatus often causes the mist or gas of the treatment fluid to scatter around. Moreover depending on the type of such coating apparatus in use, special skill for the fluid application is required.
In order to solve the above problem, a vacuum coating apparatus for wood treatment has been invented (GB2145442B). As FIG. 4 shows, the apparatus has a chamber 91 through which a material 8 is passed via an inlet port 911 and an outlet port 912, an evacuating device 92 by which the chamber 91 is subjected to a partial vacuum, and a fluid supplier 93 from which treatment fluid 7 is supplied to the chamber 91.
The material 8 is continuously fed into the chamber 91 through the inlet port 911 to be drawn out thereof through the outlet port 912. At this time, the chamber 91 is kept subjected to a partial vacuum by the evacuating device 92 so that the treatment fluid 7 is supplied into the chamber 91 from the fluid supplier 93. This causes air to flow into the chamber 91 via the clearances at the inlet port 911 and outlet port 912, resulting in an air flow 910 over the surface of the material 8. The air flow 910 removes excess fluid from the material 8 to have a thin layer coating film evenly formed over the surface thereof.
The apparatus of the above type is so constructed as to form the thin layer coating film over the material surface at high speed. The apparatus also prevents the fluid mist from scattering and requires no special skill for the fluid application. In FIG. 4, reference numerals 921, 922 designate exhaust passages, 931 and 94 designate a fluid passage, and roller guides, respectively.
With the apparatus of the known type, however, it is difficult to ensure to have the coating film thin and evenly applied over the material surface without lowering productivity and independent of the type of the treatment fluid in use, and change in the environmental temperature.
When treating the material, particularly wood, the thickness of the film to be coated thereon is often required to be 30 .mu.m or less in order to accentuate its natural grain. For such a treatment, a substantially thin film is formed over the wood surface only without filling the naturally formed vessels in the wood with the fluid. In case of coating the film so as to have relatively a large thickness, cracks occur in the film surface. Or in such a case, processing, i.e., cutting and nailing, the wood that has been treated with the fluid might also cause the film to be cracked.
A thin film can be formed over the material surface by decreasing the speed for feeding the material in the chamber. This method, however, considerably decreases the level of productivity.
When using a paint either of water-based nature or ultraviolet curing type as the treatment fluid in an apparatus of the known type, a film with desired thickness cannot be obtained in spite of decreasing the feeding speed. In case of using the ultraviolet curing paint, the viscosity of oligomer, a component thereof, cannot be lowered easily, thus preventing a sufficient reduction of the viscosity of the paint. Generally as TABLE 2 set forth hereafter shows, the viscosity of the treatment fluid is likely to vary with the temperature. The treatment fluid thus should be kept a predetermined temperature during coating. In the known apparatus, since outside air is admitted into the chamber through the inlet port and outlet port, the temperature in the chamber is likely to vary with the outside air temperature. The temperatures in the chamber in summer and winter, or early in the morning and daytime, thus, become different. This change influences the viscosity of the fluid, preventing the thin layer coating film to be evenly formed over the material surface.